Rockwell Automation 2755 High Performance VLD Scanner - Manual (Page 24)

3–11 Installation Considerations
This section explains how to calculate minimum scans per symbol
for picket fence and step ladder applications.
Picket Fence Applications
To calculate minimum scans per symbol for picket fence
applications, use this formula:
A (X–Y)
Z
S = Scans per Symbol
A =
X = Usable Beam Length at Minimum Read Distance
Y = Symbol Length (including quiet zones)
Z = Conveyor Speed
S =
(must be at least 5)
Derated Scan Rate (nominal scan rate – 5%)
Express Usable Beam Length, Symbol Length, and Conveyor Speed
in similar units. Calculations assume that the scanner and decoder
are triggered for the entire time the symbol is present and the symbol
has a 0
°
pitch. (See ”Compensating for Pitched Symbols”.)
Example 1:
A 40 mil case code label that is 1 inch tall and 4.75 inches long
(including quiet zones) is to be read in a picket fence orientation at
125 ft/min. Space is limited so the scanner must be positioned as
close as possible to the labels.
Since the 2755-LD8C1 scanner cannot read a 40 mil label until the
scanner is 10 inches away, let’s assume that the 2755-LD4C1 scanner
can read the label. At 6 inches, the LD4C1 scanner has a usable
beam length of 5.525 inches.
The following calculation converts Conveyor Speed to similar units
(inches).
125 feet/minute x 12 inch/feet x 1 minute/60second = 25 inch/seconds
To calculate minimum scans per symbol for this application, insert
these values into the formula:
A = 200 scans/second – 5% = 190 scans/second (minimum scan rate)
X = 5.525 inches
Y = 4.75 inches
Z = 25 inches/second
S = 190 scans/second x (5.525 inches – 4.75 inches) / (25 inches / second)
S = 5.89 scans
Since S (scans per symbol) is greater than 5 and the application uses
good quality labels, the 2755-LD4C1 scanner is appropriate for this
application.
Calculating Scans/Symbol